The present invention relates to brake shoe assemblies for railroad cars and particularly to spring-type brake shoe locking keys that are adapted to removably affix a brake shoe to its brake head.
In FIG. 1, there is shown a brake shoe assembly including a spring-type brake shoe locking key 110 adopted by the Association of American Railroads (A.A.R.) as an alternate standard for the standard tapered-type key. Studies have shown the tapered key to be subject to accelerated wear, which results in loss of its ability to firmly affix the brake shoe to its backing plate. As is well-known, any looseness at the brake shoe/brake head interface results in undesirable brake head wear due to vibrations typically encountered in railroad service. The alternate standard brake shoe key 110 shown in FIG. 1 is made of spring steel and firmly holds the brake shoe 136 to the brake head 138 under tension of the spring load created in the locking key as a result of the key being flexed when driven into its locking position. This spring tension of the locking key takes up play between the brake head and shoe, to prevent the brake shoe backing plate 137 from impacting the brake head in the face of such vibration as would otherwise cause impact forces to rapidly wear the brake head.
A disadvantage of this spring-type key 110 is the difficulty encountered in attempting to remove the key when the key has been driven fully into its locking position beyond the initial locking position shown in FIG. 1. In this initial locking position, an offset notch 122 intermediate the key head 114 and the end 128 of the key engages the end of the brake head carrier lug 146. The latching force of this engagement under spring tension combined with the force of friction between the key and brake head is intended to prevent the key from being dislodged in service. A pry bar is typically employed by railroad personnel to force the key out of its locking position by wedging the pry bar between the head of the key and the brake head. In the fully driven locking position of the brake shoe key, however, the key head abuts the brake head, making it difficult to wedge a pry bar therebetween. For this reason, railroad personnel have been prone to drive the brake shoe key short of its locking position, in order to facilitate subsequent removal of the key.
While the foregoing practice assures that the head of the key is spaced sufficiently from the brake head to accommodate the insertion of a pry bar therebetween, failure to drive the key at least into its initial locking position shown in FIG. 1 results in the brake shoe being affixed to the brake head with less than sufficient flexure force to prevent vibration from causing impact wear of the brake head.
This brake head wear at the interface with the brake shoe backing plate tends to further reduce the spring tension of the locking key such that continued vibration and consequent wear become progressively worse. Under such conditions, the locking key loosens to such extent that the key is prone to be dislodged in service.
For this reason, the bottom end 128 of this spring-type locking key is formed with a series of serrations. As shown in FIG. 1, these serrations are intended to engage the lower end of the brake head to prevent a loose key and/or a key driven short of its initial locking position from becoming unintentionally dislodged from its connection between the brake shoe and brake head during service, particularly in the absence of any latching force between the offset notch 122 and the brake head carrier lug 146.
It has been found in actual practice, however, that these serrations do not always perform as intended for various reasons, such as tolerance variations that arise from the many different combinations of brake head and brake shoe designs employed.
Loss of a loose brake shoe key can occur in normal service due to the key being jarred free of its connection between the brake shoe and backing plate by vibration when the serrations formed on the end of the key fail to engage the bottom of the brake head. This problem is even more acute in rotary dump service where special cars carrying coal, for example, are turned upside-down to empty the coal. In this case, gravity encourages a loose key to become dislodged from its engagement between the brake shoe and brake head. In either case, a lost key permits the brake shoe to fall off of its brake head so that in addition to a lost brake shoe, brake force normally provided by the brake shoe is not realized. In addition, the shoeless brake head can contact the wheel during a brake application and become damaged.
In rotary dump service, this problem of lost brake shoe keys is further exacerbated by reason of the fact that a dislodged key typically falls into the coal deposit. This creates a considerable expense for power plant operators in providing metal detectors to locate lost keys in the coal deposit and for the removal of these keys, especially where these lost keys represent potential damage to expensive equipment, such as pulverizing machinery etc.